42 MR. town's observations 



or even the whole of the internal surface of the chalk Echini. 

 These crystals are of a pyramidal figure, and each plate of 

 the shell of the Echinus is occupied by the base of a single 

 crystal, the size of the crystals being regulated by the size of 

 the plates, an arrangement depending, I suppose, upon some 

 law of crystallization with which I am not familiar. Now it 

 would seem that these crystals existed before the introduction 

 of the flint into the cavities of the Echinites, as I have had 

 specimens in my possession in which the crystals are pre- 

 sent, while the rest of the hollow is occupied by silex.* A 

 cast formed under these circumstances has a most remarkable 

 appearance ; for instead of its giving you a mould of the ca- 

 vity of the shell, it is a mould of the cavity formed by the 

 crystals which line the shell ; and what renders the appearance 

 still more puzzling is this, that wherever the silex is in im- 

 mediate contact with the crystals, the silex itself assumes a 

 semi-crystalline structure. 



If the various points connected with this subject were tho- 

 roughly investigated, I think it probable that the result of the 

 enquiry might develope some important suggestions with re- 

 ference to some of the conditions under which the chalk- 

 flints were deposited or aggregated. Considered zoologically, 

 the enquiry is certainly not without interest ; for our know- 

 ledge of some extinct organisms is drawn from natural moulds ; 

 and as I have shown that these moulds are sometimes modi- 

 fied by subsequent causes, all facts tending to elucidate the 

 nature and possible extent of those modifications, must be 

 looked upon as a means of guarding us against inferences of 

 a fallacious nature. 



MAGAZINE OF NATURAL HISTORY. 



JANUARY, 1840. 



A Paper has been published in the last number of Guy's Hospital Me- 

 dical Reports,^ detailing some facts connected with the development of 



1 The most characteristic specimen of this kind that I have ever exa- 

 mined, is in the rich collection of Mr. Fitch, of Norwich. 



2 Guy's Hospital Reports, No. 9, October, 1839: edited by G. H. 

 Barlow, M.A., &c., and J. R Babington, M.A., &c. 



